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TIdMessagePart

TIdMessagePart = class(TCollectionItem)

Example​

procedure DescribePart(const Part: TIdMessagePart; var Value: Variant);
begin
  Value := Part.ContentType + ' / ' + Part.ContentTransfer;
end;

Usage​

TIdMessagePart provides MIME part metadata shared by text and attachment items, including headers, content type, transfer encoding, filename and related identifiers.

Ownership and lifetime​

Normal parts belong to TIdMessage.MessageParts; clearing the collection or message makes its parts unavailable. Do not free a returned part. Headers is a read-only list reference, while assigning ExtraHeaders copies lines into the existing list rather than replacing that list.

Both list references are live and must not be freed. Avoid Part.ExtraHeaders := Part.ExtraHeaders: its setter calls the owned header list's TStrings.Assign, whose current Velox self-assignment clears before enumerating the same object, leaving the list empty.

The TCollectionItem ID/index/collection rules apply. A detached item has no owner message, so MIME default resolution and owner callbacks can differ.

Additional Technical Info​

TIdMessagePart is the common base for TIdText and TIdAttachment. It supplies MIME header metadata, field-backed filename/charset/name state, PartType classification and ResolveContentType defaults.

Do not create the base class directly. Its constructor inserts through TCollectionItem and then raises EIdCanNotCreateMessagePart when the runtime class is exactly TIdMessagePart. Obtain a concrete item from a message or construct it through an appropriate mail helper.

Header-backed versus field-backed metadata​

These properties are first-value views over the internal header list: ContentDescription, ContentDisposition, ContentID, ContentLocation, ContentTransfer and ContentType. Empty assignment removes the first matching header. If duplicates exist, a later duplicate can then become the visible value.

CharSet, FileName and Name are independent fields. Setting ContentType extracts/removes charset and name parameters and updates the fields only when extracted values are nonempty. Setting ContentDisposition similarly extracts/decodes a nonempty filename. Missing parameters do not clear stale fields, and writing a field does not update the corresponding header.

Direct edits through Headers, the convenience properties and the standalone fields can therefore diverge. Choose one controlled authoring path and read back the effective state before encoding.

Assignment and encoded state​

Native Assign copies Headers, ExtraHeaders, CharSet, FileName and Name. It does not copy parent-part linkage, callbacks or IsEncoded. Header assignment includes the header-backed metadata.

IsEncoded initializes false. A complete source search finds no later assignment to the part field in this product version, so it remains false for current native parts and should not be used as an encoding decision.

Metadata setters do not validate MIME tokens, media types, identifiers, URIs, filenames or transfer/body compatibility. Header operations can be non-transactional when callbacks/allocation fail. Parts, headers, fields and their owner collection are mutable and not thread-safe.

The source-reviewed example reads an existing borrowed part and was not executed against mail infrastructure.

External references

Created 2026-07-15